Trimmed Spearman-Kärber Method
The Trimmed Spearman-Kärber Method (S-K) is fundamentally the same as the untrimmed Spearman-Kärber Method.
Trimming is an attempt to correct for non-symmetry in the tails of the dose-effect curve, performed by deleting extreme values and using central data. This can be useful when there are unexpectedly large proportions of organisms in either tail of the distribution, i.e., many of them react at low concentrations or many fail to react at high concentrations. Hamilton (1979, 1980) studied these situations and found that a small amount of trimming resulted in a standard error for the estimated EC50 that was much smaller (i.e., more optimistic) than for other reference methods such as maximum likelihood probit or logit analysis. Excessive trimming further decreased the standard error but increased the estimated LC50, EC50, and LD50 values.
Hamilton suggested trimming of 10-20% for LC50, EC50, and LD50 estimates in cases with erratic results in the tails of the distribution, while recommending avoidance of trimming for data with a regular distribution (Environment Canada, 2005).
Analysis Results of ToxGenie’s Trimmed Spearman-Kärber Method
ToxGenie automatically calculates the trimming level based on the characteristics of the input data. To compare ToxGenie’s analysis results, both the trimmed and untrimmed Spearman-Kärber Methods were applied using the same data as in the US EPA Report (EPA 600/4-85-013, 3rd edition, 1985). The results are shown in the Spearman-Kärber Method.
Due to differences in the number of decimal places used in mathematical calculations and the criteria for rounding, the results are not entirely identical to those of the US EPA analysis. However, they demonstrate highly accurate calculations. For point estimation analysis, ToxGenie recommends the Logit Method for the US EPA data. However, if the user does not apply the recommended analysis method, ToxGenie can also perform the analysis using a user-specified method.
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